Oxidation resistant organic hydrogen getters

ABSTRACT

A composition for removing hydrogen from an atmosphere, comprising a mixture of a polyphenyl ether and a hydrogenation catalyst, preferably a precious metal catalyst, and most preferably Pt. This composition is stable in the presence of oxygen, will not polymerize or degrade upon exposure to temperatures in excess of 200° C., or prolonged exposure to temperatures in the range of 100-300° C.

STATEMENT OF GOVERNMENT INTEREST

[0001] This invention was made with Government support under contractno. DE-AC04-94AL85000 awarded by the U.S. Department of Energy to SandiaCorporation. The Government has certain rights in the invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] Not applicable.

BACKGROUND OF THE INVENTION

[0003] The present invention is directed to a hydrogen getter materialcomprising a mixture of a polyphenyl ether and a hydrogenation catalyst,preferably a metal selected from Group VIII of the Periodic Table of theElements.

[0004] In many applications the presence of hydrogen and its isotopes,arising from various chemical and electrochemical corrosion reactions,can be detrimental. Hydrogen can evolve from corrosion by atmosphericgases, by stray electric currents, from electronic devices, which caninclude batteries, operating in normal or abnormal condition, and fromleaky hydrogen piping. The accumulation of hydrogen can present asignificant fire and/or explosion hazard particularly in sealedcomponents where special precautions may need to be taken to preventdangerously high internal pressures from developing. Hydrogen corrosionis a particular problem in downhole fiber optic systems. Hydrogen attackin fiber optic systems reduces the optical transmission efficiency ofthese devices.

[0005] It has long been known that hydrogen absorbing materials, knownas getters, can be used to counteract hydrogen accumulation. Ayers etal. discuss the use of active metals such as zirconium or titanium, andalloys thereof in U.S. Pat. No. 4,512,721. These metals are capable ofmaintaining low hydrogen partial pressures but have the disadvantage ofrequiring high temperatures for initial activation and/or ongoingoperation (generally >300° C.) because of the necessity to diffusesurface contaminants into the bulk metal thereby providing a freshsurface for continued hydrogen absorption.

[0006] Labaton, in U.S. Pat. No. 4,886,048, describes another means forremoving hydrogen by reacting the hydrogen with oxygen to form water, inthe presence of a noble metal catalyst such as palladium, and trappingthe water on a water absorbing material such as a molecular sieve.However, hydrogen getters of this type are expensive, bulky, limited bythe availability of oxygen, and capable of causing a detonation ifimproperly formulated.

[0007] Conventional hydrogen getters, such as those described in theabove-referenced patents are expensive, can require special operatingconditions such as high temperature regimes or ancillary reactants inorder to maintain low hydrogen partial pressures, generally will notwork well or at all in the presence of water, may require the presenceof oxygen, be poisoned by oxygen, and may pose significant safetyhazards, including fire and explosion if handled improperly, for exampleexposure to air.

[0008] In order to overcome the aforementioned problems withconventional hydrogen getters, Shepodd in U.S. Pat. Nos. 5,703,378,5,837,158 and 5,624,598 discloses and describes organic getter systemsthat employ unsaturated organic compounds combined (i.e., organiccompounds that contain carbon-carbon double or triple bonds) with noblemetal catalysts as hydrogen getter materials. While these organic gettersystems have been shown to work well for temperatures below about 200°C., because of the presence of double or triple bonds in these prior arthydrogen getters they begin to degrade appreciably at temperatures above200° C. and slowly over time at temperatures above about 150° C.Moreover, the unsaturated organic compounds will polymerize at elevatedtemperatures, thereby impairing their performance as hydrogen getters.However, there is a need for a hydrogen getter material that is capableof gettering hydrogen in the temperature range of 150-300° C. This needis acutely felt in the oil well industry where downhole fiber opticsystems are used. Hydrogen present in the downhole environment attacksthe fiber optic reducing its transmission efficiency. This temperaturerange (150-300° C.) is well above the effective operating range of priorart unsaturated organic hydrogen getters but below that where metallicgetters can be used.

SUMMARY OF THE INVENTION

[0009] Accordingly, the present invention is directed to a material foreffectively removing hydrogen from an atmosphere at temperatures in therange of about 150-300° C.; a hydrogen getter. The hydrogen gettermaterials disclosed herein provide for removal of hydrogen in thepresence of contaminants such as common atmospheric gases, water, watervapor, and oil mists.

[0010] The hydrogen getter material of the present invention comprises amixture of a polyphenyl ether and hydrogenation catalyst, preferably ametal selected from Group VIII of the Periodic Table of the Elements,hereinafter a precious metal, and most preferably Pt. In formulating thegetter material, an inert binder material can be used to control thephysical state of the getter material. In contrast to prior art organichydrogen getter materials, the present hydrogen getter material isstable in the presence of oxygen, will not polymerize upon exposure totemperatures in excess of 200° C., or prolonged exposure to temperaturesin the range of 100-300° C.

[0011] The novel hydrogen getter material disclosed herein can be usedto efficiently removing hydrogen from mixtures of hydrogen/inert gas(e.g., He, Ar, N2), hydrogen/ammonia atmospheres, such as may beencountered in heat exchangers, and hydrogen/carbon dioxide atmospheres.Water vapor and common atmospheric gases have no adverse effect on theability of these getter materials to absorb hydrogen. Liquid water doesnot have an adverse effect on the efficiency of these hydrogen gettersexcept that, if submerged, the reaction with hydrogen can be limited bythe rate at which hydrogen can diffuse through liquid water.

DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention provides a composition for absorbinghydrogen from an atmosphere. This novel hydrogen absorbing compositionis stable in the presence of oxygen, will not polymerize upon exposureto temperatures in excess of 200° C., or prolonged exposure totemperatures in the range of 100-300° C. The hydrogen absorbingcomposition comprises a mixture of a polyphenyl ether and ahydrogenation catalyst, preferably a precious metal catalyst, and mostpreferably Pt. Throughout the written description, the art-recognizedterm “hydrogen getter” will be used to denote the inventive hydrogenabsorbing composition.

[0013] In contrast to prior art organic hydrogen getter materials,wherein hydrogen reacts with carbon-carbon double or triple bonds in thepresence of a catalyst (Shepodd, op. cit.) to produce an alkane, thepresent invention employs a phenyl moiety in the form of a polyphenylether and hydrogen is added to the phenyl rings.

[0014] As represented in formula I, polyphenyl ethers are aromaticethers that consist, generally, of basic structural units comprisingalternating phenyl groups and oxygen atoms.

[0015] The value of n, number of basic structural units, can be variedto provide desired properties. Hydrogenation of a polyphenyl ether inthe presence of a hydrogenation catalyst is a heterogeneous reaction. Aswill be readily appreciated, in order to achieve the highest degree ofeffectiveness in removing hydrogen it is desirable that the polyphenylethers used to prepare the hydrogen getter material be mobile enough toprovide for intimate contact with the catalyst. Thus it is preferredthat the value of n be greater than 3 in unsubstituted polyphenyl ethersand most preferably in the range of 4-7. However, effective hydrogengetter materials can be prepared from polyphenyl ethers having a valueof n less than 3 (i. e., diphenyl ethers that are solid at roomtemperature) by mixing a catalyst material with the liquid produced bymelting the ether and comminuting the solid mixture.

[0016] Polyphenyl ethers are thermally stable and resistant to oxidationup to temperatures of 300° C. Moreover, at temperatures below 200° C.polyphenyl ethers have very low vapor pressures (Synthetic Lubricantsand High-Performance Functional Fluids, 2^(nd) edition, Rudnick, L. andShubkin, R., Marcel Dekker, Inc. 1999). Unlike prior art gettermaterials, polyphenyl ethers will not polymerize at elevatedtemperatures (i.e., T greater than about 100° C.) when combined with aprecious metal catalyst.

[0017] The getter materials can be formulated by mixing or blendingtogether a polyphenyl ether and a precious metal hydrogenation catalystto form a powder or paste, depending upon the ratio of components.Useful hydrogenation catalysts include Pd, Pt, Au, Rh, and Ru orcombinations or alloys thereof. A preferred hydrogenation catalyst isPt. The hydrogenation catalyst can either be added as a metal powder; asa supported catalyst, wherein the catalyst is dispersed on an inertmaterial, or support, such as activated carbon, aluminum oxide, orbarium carbonate; or as a metallic salt that is reduced to the metalupon contact with hydrogen. Typically, about 5-50 wt % of a supportedcatalyst containing about 1-10 wt % metal, or an equivalent weight ofmetal powder, is combined with the ether.

[0018] Binders or fillers that are inert with respect to reaction withhydrogen can be included in the hydrogen getter formulation to tailorthe properties of the getter material and can take many forms. Thesimple addition of inert polymers, thixotropic agents, minerals, carbonpowders, or finely divided silica can be used to mediate the reactionrate, act as a heat sink, and modify the physical properties of thegetter. If a binder is used, it is preferred that the inert bindermaterial be present in a concentration of from about 20-70 wt %. Itshould be noted that the hydrogenation catalyst can also act as abinder, however, additional catalyst will speed the reaction but reducethe hydrogen-absorbing capacity per unit mass of formulated getter andincrease the cost.

[0019] As discussed above, these getter materials are capable ofremoving hydrogen from gas mixtures containing hydrogen. Of particularinterest is the removal of hydrogen from hydrogen/oxygen mixtures. Itwill be appreciated that hydrogen and oxygen can form explosive mixturesover the concentration range of about 6 to about 70 vol % H₂. Noblemetal hydrogenation catalysts, especially when heated can initiateignition of a hydrogen/oxygen gas mixture within the explosiveconcentration range. However, it is believed that two reactions occursimultaneously in the getter material during the course of hydrogenabsorption in a H₂/O₂ atmosphere. One is the combination of hydrogen andoxygen in the presence of the hydrogenation catalyst to make water. Thesecond is the hydrogenation of the phenyl rings of the polyphenyl ether.The generation of heat with the consequent heating of surroundingmaterial occurs both as hydrogen and oxygen combine exothermically andrapidly in the presence of a catalyst to make water as well as aconsequence of the hydrogenation of the unsaturated carbon-carbon bondsin the organic polymer molecule. If means to control the reaction rateare not provided, such as that described in the present invention,detonation of the hydrogen/oxygen gas mixture can take place. Theinventors have discovered that in addition to their ability to absorbhydrogen, the polymer getter materials of the present invention have anadditional advantage in that they can moderate the explosively rapidreaction that can take place between hydrogen and oxygen, in thepresence of a catalyst, making a detonation unlikely. It is believedthat safe removal of hydrogen from H₂/O₂ mixtures is possible becausehydrogenation of the phenyl rings provides a competitive reaction to thepotentially explosive H₂/O₂ combination reaction. Further, the gettermaterial provides a heat sink to prevent localized overheating, and adiluting medium to restrict access of hydrogen and oxygen to thecatalyst thereby slowing the H₂/O₂ reaction. Moreover, the gettermaterial can switch back and forth between acting as a catalyst forhydrogen/oxygen recombination and hydrogen absorber without any loss inefficacy.

[0020] The present invention now will be described more fully by way ofvarious examples illustrative of the invention. This invention may,however, be embodied in many different modifications that will beobvious to those of skill in the art without departing from theinvention set forth in the claims.

[0021] Table 1 identifies various formulations that were prepared toillustrate the invention. The formulations shown in Table 1 were allprepared by mixing the components together to provide a dispersion ofthe catalyst throughout the getter material. Because of theheterogeneous nature of the hydrogenation reaction and the limitedmobility of the polyphenyl ethers, it is important that the catalyst beuniformly dispersed throughout the getter material in order to providemaximum efficiency. In this regard, while hand mixing will yield useableworking formulations mechanical mixing is preferred to fully anduniformly disperse the catalyst. Different mechanical mixers such asblenders, attritors, or kneaders are effective depending upon theviscosity of the starting materials and product. Liquids such as organicsolvents or water can be added as processing aids, however, were notused to prepare the formulations shown in Table 1. TABLE 1 FormulaComposition Physical State A 53 g. 1% Pt/C, 73 g Santovac ® 5* Paste B45 g carbon black, 22.5 g 1% Pt/C Powder 23.1 g Santovac ® 5 C 20 gcarbon black, 10 g Pt/C Powder 20 g Santovac ® 5 D 20 g carbon black, 10g 10% Pt/C Powder 20 g Santovac ® 5 E 20 g carbon black, 10 g 1% Pt/CPowder 20 g OS-138#

[0022] Each of the formulations shown in Table 1 was placed in areactor, the reactor was heated to a predetermined temperature and about10 Torr of hydrogen was admitted to the reactor. Calculated reactionrates, based on pressure measurements, are shown in Table 2. TABLE 2(PH₂ ≈ 10 TORR) Formula T (C) Reaction Rate (std cc/g-s) A 100 3.0 e−3200 7.8 e−3 250 1.1 e−3 300 3.8 e−2 B 21 3.5 e−5 41 9.3 e−5 62 1.5 e−483 2.8 e−4 100 2.4 e−4 C 23 9.0 e−6 55 7.6 e−5 82 2.0 e−4 108 3.2 e−4 D23 1.3 e−3 54 1.0 e−3 80 1.5 e−3 105 3.2 e−3 E 24 6.5 e−6 54 6.0 e−5 791.7 e−4 104 2.7 e−4

[0023] While the hydrogen getter composition is intended to function attemperatures where other hydrogen getter materials fail (i.e., T>200°C.), careful examination of the data in Table 2 shows that thepolyphenyl/Pt getter material can be made to work effectively attemperatures much less than that by excess loading of the hydrogencatalyst material. It has been found that for metal catalystconcentrations greater than about 0.5 wt % the hydrogen getteringefficiency at temperatures below 200° C. is markedly increased. Thiseffect is shown in Table 3 where the ratio of the rate of hydrogenabsorption for a ten-fold increase in Pt catalyst loading is given forvarious temperatures ranging from about 25° C. to about 100° C. TABLE 3Ratio of H₂ absorption rates T (° C.) 143 23.4 13.3 54.4 7.2 81.2 9.7106.8

[0024] As the temperature increases the increases the ratio of hydrogenabsorption rates begins to asymptotically approach the Ptcatalyst-loading ratio of 10.

We claim:
 1. A composition for absorbing hydrogen, comprising: a mixtureof a polyphenyl ether and a hydrogenation catalyst.
 2. The compositionof claim 1, wherein the polyphenyl ether is comprised of 4 to 7 basicstructural units.
 3. The composition of claim 1, wherein thehydrogenation catalyst is a precious metal or a metallic salt thereof.4. The composition of claim 3, wherein the hydrogenation catalyst is Pt.5. The composition of claim 4, wherein the hydrogenation catalyst ispresent at a concentration of from about 0.5 to 5 wt %.
 6. Thecomposition of claim 1, wherein the hydrogenation catalyst is supportedon a porous solid.
 7. The composition of claim 6, wherein the poroussolid is activated carbon, aluminum oxide, or barium carbonate, orcombinations thereof.
 8. The compositin of claim 1, wherein theconcentration of supported hydrogenation catalyst is from about 5-50 wt% of the supported catalyst containing about 1-10 wt % metal.
 9. Thecomposition of claim 1, further including a binder or filler.
 10. Thecomposition of claim 9, wherein the binder or filler is an inertpolymer, a thixotropic agent, a mineral, a carbon powder, or finelydivided silica.
 11. The composition of claim 10, wherein the binder orfiller is present at a concentration of from about 20-70 wt %.